1. Field of the Invention
The invention is related to the field of communications, and in particular, to a communication system with two antennas and two receivers.
2. Description of the Prior Art
People and businesses are demanding higher bandwidths from their communication providers. Consequently, the communication providers are looking for ways to increase the bandwidth of their systems using broadband technologies. Broadband technologies are generally referred to as systems that deliver a bandwidth at or above 64 kbps. Broadband technologies can communicate over downstream channels and upstream channels. The customer receives data from another device or system over the downstream channels. The customer transmits data to another device or system over the upstream channels.
Broadband Wireline Systems
One example of a broadband technology is Digital Subscriber Line (DSL) service. DSL service can carry both voice signals and data signals at the same time in both directions. DSL service also can carry call information and customer data. DSL service is typically comprised of twisted-pair wires that connect a customer to a central office. The central office comprises a Digital Subscriber Line Access Multiplexer (DSLAM) that provides the DSL service to the customer. Unfortunately, the speed of DSL service is limited by the distance between the customer and the DSLAM. Customers located too far from the DSLAM may not be able to receive high-speed service. Also, there may not be enough customers within a particular area to make it economical to install a DSLAM. The quality of DSL service is also limited by the quality of the copper wire that connects the customer to the DSLAM. Furthermore, DSL service does not work over Digital Loop Carrier (DLC) lines.
Another broadband technology is cable modem service. The cable modem communicates with a device or system over a coaxial cable. The coaxial cable is typically the same coaxial cable used to receive cable television. The cable modem service can be one-way or two-way. In a two-way system, the coaxial cable carries both the upstream channels and the downstream channels. In a one-way system, the cable modem receives data on the downstream channels over the coaxial cable and transmits data on the upstream channels over a phone line. Unfortunately, the cable modem uses up valuable bandwidth on the phone line in the one-way system. Also, the upstream bandwidth is small over a phone line.
Broadband Wireless Systems
Another broadband technology is broadband wireless service. Customers that subscribe to broadband wireless service communicate with a head end. In a one-way wireless system, a transmitter antenna for the head end broadcasts wireless signals to the customer on the downstream channels. The transmitter antenna is a satellite antenna or a land-based antenna. The customer transmits data to the head end over another medium, such as a phone line or a cable modem, on the upstream channels. One example of a one-way wireless system is a Digital Satellite System (DSS) from DIRECTV.
A specific type of broadband wireless system communicates over Multichannel Multipoint Distribution Service (MMDS) frequencies and Multipoint Distribution Service (MDS) frequencies. The MMDS frequencies range from 2500 MHz to 2686 MHz. The MDS frequencies range from 2150 MHz to 2162 MHz. In a typical MMDS system, the bandwidth of the upstream channels is about 6 MHz. The upstream bandwidth is divided into subchannels. Each subchannel has a bandwidth of 200 kHz. In other examples, each subchannel has a bandwidth of 166 KHz.
A head end manages the upstream and downstream channels with the customer. The head end also interfaces the customer with communication networks such as the Internet. The head end includes a base antenna comprised of a transmitter antenna and one or more receiver antennas. MMDS requires a line of sight between devices that are communicating. Therefore, the antennas are placed on a high building or a mountain to establish lines of sight with the customers.
The transmitter antenna is omni-directional and broadcasts data from the head end to the customers on the downstream channels. In a two-way wireless system, the receiver antennas are positioned to receive MMDS signals transmitted from customers to the head end on the upstream channels. Each receiver antenna is positioned to receive MMDS signals from customers located within a certain area. The areas formed by the antennas are referred to as sectors. The sectors have designated frequency ranges or designated channels.
The head end is comprised of an upstream manager and a downstream manager that control transmissions on the upstream channels and the downstream channels, respectively. As stated above, the upstream channels and the downstream channels are divided into subchannels. One upstream subchannel is a contention channel reserved for signaling, while the remaining subchannels are bearer channels.
In the broadband wireless system, a wireless broadband router is located at a customer premises. The wireless broadband router communicates with the upstream manager and the downstream manager to exchange data. The upstream manager generally operates the channels and/or subchannels in four states: idle, contention, polling, and dedicated. In the idle state, the channels are idle. In the contention state, the upstream manager generates and transmits control signals over one or more subchannels.
For the polling and dedicated states, the upstream manager polls numerous wireless broadband routers to allocate use of the subchannels. Polling is a round robin process to determine which wireless broadband router has access to a subchannel. The upstream manager maintains a queue of the active wireless broadband routers to determine which wireless broadband router is next to transmit over a subchannel for a period of time. The upstream manager keeps an inventory of open subchannels and waiting wireless broadband routers in the queue.
Sectorization
Other communication systems such as cellular communication systems and personal communication service systems use sectorization. These systems use 60, 90, and 120 degrees sectors to increase capacity. Military radars use apertures as small as 2 degrees.
In this broadband wireless system, the downstream channels handle more capacity than upstream channels due to licensing requirements for MMDS. In order to increase upstream capacity, the broadband wireless system receives wireless signals from 45 degree sectors. However, the 45 degree sectors in the broadband wireless system do not have the capacity to support numerous users in a metropolitan area.
Another problem with the 45 degree sector is a problem of re-reflection. FIG. 7 depicts an illustration of a sector map of the broadband wireless system in the prior art. The broadband wireless system provides communication services to users in 8 45 degree sectors. These 8 sectors are sector #1 710, sector #2 720, sector #3 730, sector #4 740, sector #5 750, sector #6 760, sector #7 770, and sector #8 780. The sector #1 710, the sector #3 730, the sector #5 750, and the sector #7 770 use a first set of MMDS frequencies. The sector #2 720, the sector #4 740, the sector #6 760, and the sector #8 780 use a second set of MMDS frequencies. The sector #1 710 includes a receiver antenna 712, an obstruction 714, and a wireless broadband router 716. The obstruction 714 could be any object such as other communication towers or trees. The sector #3 730 includes a receiver antenna 732 and a wireless broadband router 736. The other receiver antennas and various obstructions are not shown for the sake of simplicity. Numerous wireless broadband routers are also not shown for the sake of simplicity.
A first re-reflection problem is the wireless broadband router 736 in the sector #3 730 transmits wireless signals that reflect off the obstruction 714 into the receiver antenna 712 in the sector #1 710. Thus, due to the operation of the sector #1 710 and the sector #3 730 in the same first frequency set, the wireless signals of the wireless broadband router 736 in the sector #3 730 interferes in the operation of the sector #1 710. Another problem is the wireless broadband router 716 located behind the obstruction 714 uses increased power to overcome the obstruction 714. The power that the wireless broadband router 716 uses is limited by the hardware and licensing restrictions. The increased power creates greater interference and noise for the neighboring sectors.
Unfortunately, the broadband wireless systems that use 45 degree sectors is not effective in supporting numerous users in a metropolitan area. Also, the broadband wireless systems that use 45 degree sectors does not operate effectively due to problems associated with re-reflection.